A conventional local oscillator disclosed in Patent Document 1 is described with reference to FIG. 9. In FIG. 9, conventional oscillator 10 includes oscillation circuit 2 and resonance circuit 30 connected to this oscillation circuit 2. Oscillation circuit 2 and resonance circuit 30 are connected in a loop with PLL circuit 60.
Resonance circuit 30 includes inductor elements 20a and 20b connected to oscillation circuit 2. Inductor elements 20a and 20b are connected to series, and their contact point is connected to VCC. Resonance circuit 30 includes first variable capacitance section 330 connected in parallel to inductor elements 20a and 20b, and second variable capacitance section 430 connected in parallel to this first variable capacitance section 330. First variable capacitance section 330 is connected in parallel to inductor elements 20a and 20b, and includes variable capacitive elements 310a and 210b that change a capacity value based on control voltage supplied from control voltage input terminal 35 and reference voltage VREF supplied from reference voltage supply 500 and capacitive elements 311a and 311b connected in series to this variable capacitive element 310. Variable capacitive elements 310a and 310b are connected in series, and their contact point is connected to reference voltage supply 500. Variable capacitive element 310a and capacitive element 311a, and variable capacitive element 310b and capacitive element 311b are connected in series, respectively. Their contact points are connected to control voltage input terminal 35. Variable capacitive element 310a and capacitive element 311a, and variable capacitive element 310b and capacitive element 311b configure variable capacitance circuits 31a band 31b, respectively.
Second variable capacitance section 430 includes first capacitance switch circuits 41a and 41b connected in parallel to inductor elements 20a and 20b, and second capacitance switch circuits 42a and 42b connected in parallel to these first capacitance switch circuits 41a and 41b. First capacitance switch circuit 41a includes capacitive element 410a and first switch 411a connected in series to this capacitive element 410a. This first switch 411a is switched between on and off by a signal supplied from first capacitance control terminal 413. In the same way, first capacitance switch circuit 41b includes capacitive element 410b and first switch 411b connected in series to this capacitive element 410b. This first switch 411b is switched on and off by a signal supplied from first capacitance control terminal 413. First switches 411a and 411b are connected in series, and their contact point is connected to GND. Second capacitance switch circuit 42a includes capacitive element 420a and second switch 421a connected in series to this capacitive element 420a. This second switch 21a is switched between on and off by a signal supplied from second capacitance control terminal 423. In the same way, second capacitance switch circuit 42b includes capacitive element 420b and second switch 421b connected in series to this capacitive element 420b. This second switch 421b is switched between on and off by a signal supplied from second capacitance control terminal 423. Second switches 421a and 421b are connected in series, and their contact point is connected to GND.
A characteristic of oscillation frequency of local oscillator 10 as configured above is described next with reference to FIGS. 9 and 10.
In FIG. 9, the frequency of the oscillation signal from local oscillator 10 is the resonance frequency of resonance circuit 30. This resonance frequency is determined by first variable capacitance section 330, second variable capacitance section 430, and inductors 20a and 20b. The capacity values of variable capacitive elements 310a and 310b provided in first variable capacitance section 330 are determined by the reference voltage supplied from reference voltage supply 500 and the control voltage supplied from control voltage input terminal 35. The capacity value of second variable capacitance section 430 is determined by how many of capacitive elements 410a and 410b and capacitive elements 420a and 420b are connected in parallel by the on-and-off combination of first switches 411a and 411b and second switches 421a and 421b. 
A control signal output from frequency-adjusting circuit 670 of PLL 600 is used for controlling first switches 411a and 411b and second switches 421a and 421b. This frequency-adjusting circuit 670 compares the frequency difference between output signal fDIV of oscillation-frequency divider 610 and output signal fREF of reference-frequency divider 630, and determines a capacity value that minimizes the frequency difference between fDIV and fREF. Frequency-adjusting circuit 670 then examines the combination of capacitive elements 410a and 410b and capacitive elements 420a and 420b, and controls first switches 411a and 411b and second switches 421a and 421b via first and second capacitance control terminals 413 and 423. This structure achieves a broad oscillation frequency range.
FIG. 10 is a graph illustrating frequency characteristic, which shows the relationship between first variable capacitance control voltage (VT) applied to control voltage input terminal 35 and oscillation frequency (fOSC) of local oscillator 10 in FIG. 9. Bands 1 to 4 show the characteristic of VT vs. fOSC when first switches 411a and 411b and second switches 421a and 421b are turned on and off in four combinations.
In Band 1, in which fOSC is the lowest, first switches 411a and 411b and second switches 421a and 421b are all turned on, and thus the capacity value of second variable capacitance section 430 is the sum of capacitive elements 410a and 410b and capacitive elements 420a and 420b. On the other hand, in Band 4, in which fOSC is the highest, first switches 411a and 411b and second switches 421a and 421b are all turned off, so the capacity value of second variable capacitance section 430 is approximately 0. In Bands 2 and 3, in which fOSC against VT is between Band 1 and Band 4, only one of first switches 411a and 411b and second switches 421a and 421b is turned on. The capacity value of second variable capacitance section 430 becomes capacity value CSW1 of capacitive element 410a or 410b, or capacity value CSW2 of capacitive element 420a or 420b. If the capacity values of capacitive elements 410a and 410b and capacitive elements 420a and 420b are CSW1>CSW2, the characteristic of Band 2 is that only first switches 411a and 411b are turned on, and the characteristic of Band 3 is that only second switches 421a and 421b are turned on.
Oscillation frequencies fOSC1 to fOSC4 of Bands 1 to 4 are expressed by Formulae 1 to 4.
                              f                      OSC            ⁢                                                  ⁢            1                          =                              1                          2              ⁢              π              ⁢                                                L                  ⁢                                      {                                                                  (                                                                              C                                                          SW                              ⁢                                                                                                                          ⁢                              1                                                                                +                                                      C                                                          SW                              ⁢                                                                                                                          ⁢                              2                                                                                                      )                                            +                                                                        C                          ⁢                                                                                                          ⁢                                                      1                            ·                                                          C                              VT                                                                                                                                                            C                            ⁢                                                                                                                  ⁢                            1                                                    +                                                      C                            VT                                                                                                                }                                                                                =                      1                          2              ⁢              π              ⁢                                                L                  ⁡                                      (                                                                                            3                          2                                                ⁢                                                  C                                                      SW                            ⁢                                                                                                                  ⁢                            1                                                                                              +                                                                        C                          ⁢                                                                                                          ⁢                          1                          ⁢                                                      (                                                                                          α                                ⁢                                                                                                                                  ⁢                                                                  V                                  T                                                                                            +                              β                                                        )                                                                                                                                C                            ⁢                                                                                                                  ⁢                            1                                                    +                                                      α                            ⁢                                                                                                                  ⁢                                                          V                              T                                                                                +                          β                                                                                      )                                                                                                          [                  Formula          ⁢                                          ⁢          1                ]                                          f                      OSC            ⁢                                                  ⁢            2                          =                              1                          2              ⁢              π              ⁢                                                L                  ⁢                                      {                                                                  (                                                  C                                                      SW                            ⁢                                                                                                                  ⁢                            1                                                                          )                                            +                                                                        C                          ⁢                                                                                                          ⁢                                                      1                            ·                                                          C                              VT                                                                                                                                                            C                            ⁢                                                                                                                  ⁢                            1                                                    +                                                      C                            VT                                                                                                                }                                                                                =                      1                          2              ⁢              π              ⁢                                                L                  ⁡                                      (                                                                  C                                                  SW                          ⁢                                                                                                          ⁢                          1                                                                    +                                                                        C                          ⁢                                                                                                          ⁢                          1                          ⁢                                                      (                                                                                          α                                ⁢                                                                                                                                  ⁢                                                                  V                                  T                                                                                            +                              β                                                        )                                                                                                                                C                            ⁢                                                                                                                  ⁢                            1                                                    +                                                      α                            ⁢                                                                                                                  ⁢                                                          V                              T                                                                                +                          β                                                                                      )                                                                                                          [                  Formula          ⁢                                          ⁢          2                ]                                          f                      OSC            ⁢                                                  ⁢            3                          =                              1                          2              ⁢              π              ⁢                                                L                  ⁢                                      {                                                                  (                                                  C                                                      SW                            ⁢                                                                                                                  ⁢                            2                                                                          )                                            +                                                                        C                          ⁢                                                                                                          ⁢                                                      1                            ·                                                          C                              VT                                                                                                                                                            C                            ⁢                                                                                                                  ⁢                            1                                                    +                                                      C                            VT                                                                                                                }                                                                                =                      1                          2              ⁢              π              ⁢                                                L                  ⁡                                      (                                                                                            1                          2                                                ⁢                                                  C                                                      SW                            ⁢                                                                                                                  ⁢                            1                                                                                              +                                                                        C                          ⁢                                                                                                          ⁢                          1                          ⁢                                                      (                                                                                          α                                ⁢                                                                                                                                  ⁢                                                                  V                                  T                                                                                            +                              β                                                        )                                                                                                                                C                            ⁢                                                                                                                  ⁢                            1                                                    +                                                      α                            ⁢                                                                                                                  ⁢                                                          V                              T                                                                                +                          β                                                                                      )                                                                                                          [                  Formula          ⁢                                          ⁢          3                ]                                                          ⁢                              f                          OSC              ⁢                                                          ⁢              4                                =                                    1                              2                ⁢                π                ⁢                                                      L                    ⁢                                                                  C                        ⁢                                                                                                  ⁢                                                  1                          ·                                                      C                            VT                                                                                                                                                C                          ⁢                                                                                                          ⁢                          1                                                +                                                  C                          VT                                                                                                                                          =                          1                              2                ⁢                π                ⁢                                                      L                    ⁢                                                                  C                        ⁢                                                                                                  ⁢                        1                        ⁢                                                  (                                                                                    α                              ⁢                                                                                                                          ⁢                                                              V                                T                                                                                      +                            β                                                    )                                                                                                                      C                          ⁢                                                                                                          ⁢                          1                                                +                                                  α                          ⁢                                                                                                          ⁢                                                      V                            T                                                                          +                        β                                                                                                                                                    [                  Formula          ⁢                                          ⁢          4                ]            
Frequency control sensitivity dfOSC1/dVT to dfOSC4/dVT, which are gradients of VT vs. fOSC in bands 1 to 4 are expressed by the following formulae 5 to 8.
                                          ⅆ                          f                              OSC                ⁢                                                                  ⁢                1                                              /                      ⅆ                          V              T                                      =                                                            -                α                            ·              C                        ⁢                                                  ⁢                          1              2                                                                                            4                  ⁢                  π                  ⁢                                      L                                    ⁢                                                                                    C                        ⁢                                                                                                  ⁢                        1                                            +                                              α                        ⁢                                                                                                  ⁢                                                  V                          T                                                                    +                      β                                                                                                                                                                {                                                                                            3                          2                                                ⁢                                                                              C                                                          SW                              ⁢                                                                                                                          ⁢                              1                                                                                ⁡                                                      (                                                                                          C                                ⁢                                                                                                                                  ⁢                                1                                                            +                                                              α                                ⁢                                                                                                                                  ⁢                                                                  V                                  T                                                                                            +                              β                                                        )                                                                                              +                                              C                        ⁢                                                                                                  ⁢                        1                        ⁢                                                  (                                                                                    α                              ⁢                                                                                                                          ⁢                                                              V                                T                                                                                      +                            β                                                    )                                                                                      }                                                        3                    2                                                                                                          [                  Formula          ⁢                                          ⁢          5                ]                                                      ⅆ                          f                              OSC                ⁢                                                                  ⁢                2                                              /                      ⅆ                          V              T                                      =                                                            -                α                            ·              C                        ⁢                                                  ⁢                          1              2                                                                                            4                  ⁢                  π                  ⁢                                      L                                    ⁢                                                                                    C                        ⁢                                                                                                  ⁢                        1                                            +                                              α                        ⁢                                                                                                  ⁢                                                  V                          T                                                                    +                      β                                                                                                                                                                {                                                                                            C                                                      SW                            ⁢                                                                                                                  ⁢                            1                                                                          ⁡                                                  (                                                                                    C                              ⁢                                                                                                                          ⁢                              1                                                        +                                                          α                              ⁢                                                                                                                          ⁢                                                              V                                T                                                                                      +                            β                                                    )                                                                    +                                              C                        ⁢                                                                                                  ⁢                        1                        ⁢                                                  (                                                                                    α                              ⁢                                                                                                                          ⁢                                                              V                                T                                                                                      +                            β                                                    )                                                                                      }                                                        3                    2                                                                                                          [                  Formula          ⁢                                          ⁢          6                ]                                                      ⅆ                          f                              OSC                ⁢                                                                  ⁢                3                                              /                      ⅆ                          V              T                                      =                                                            -                α                            ·              C                        ⁢                                                  ⁢                          1              2                                                                                            4                  ⁢                  π                  ⁢                                      L                                    ⁢                                                                                    C                        ⁢                                                                                                  ⁢                        1                                            +                                              α                        ⁢                                                                                                  ⁢                                                  V                          T                                                                    +                      β                                                                                                                                                                {                                                                                            1                          2                                                ⁢                                                                              C                                                          SW                              ⁢                                                                                                                          ⁢                              1                                                                                ⁡                                                      (                                                                                          C                                ⁢                                                                                                                                  ⁢                                1                                                            +                                                              α                                ⁢                                                                                                                                  ⁢                                                                  V                                  T                                                                                            +                              β                                                        )                                                                                              +                                              C                        ⁢                                                                                                  ⁢                        1                        ⁢                                                  (                                                                                    α                              ⁢                                                                                                                          ⁢                                                              V                                T                                                                                      +                            β                                                    )                                                                                      }                                                        3                    2                                                                                                          [                  Formula          ⁢                                          ⁢          7                ]                                                      ⅆ                          f                              OSC                ⁢                                                                  ⁢                4                                              /                      ⅆ                          V              T                                      =                                                            -                α                            ·              C                        ⁢                                                  ⁢                          1              2                                            4            ⁢            π            ⁢                          L                        ⁢                                                            C                  ⁢                                                                          ⁢                  1                                +                                  α                  ⁢                                                                          ⁢                                      V                    T                                                  +                β                                      ⁢                                          {                                  C                  ⁢                                                                          ⁢                  1                  ⁢                                      (                                                                  α                        ⁢                                                                                                  ⁢                                                  V                          T                                                                    +                      β                                        )                                                  }                                            3                2                                                                        [                  Formula          ⁢                                          ⁢          8                ]            
Next, the relationship of frequency control sensitivity dfOSC1/dVT to dfOSC4/dVT of bands 1 to 4, based on formulae 5 to 8, can be expressed by formula 9.dfOSC4/dVT>dfOSC3/dVT>dfOSC2/dVT>dfOSC1/dVT  [Formula 9]
whereas, a value of each element of resonance circuit 30 in the above Formula 1 to Formula 9 is:
L: Inductance of inductor elements 20a and 20b; 
C1: Capacity value of capacitive elements 311a and 311b; 
CVT: Capacity value when control voltage VT is applied to the control voltage input terminals of variable capacitive elements 310a and 310b (=αVt+β, α and β are coefficients simplifying variable capacity value CVT against a change in control voltage); and
CSW1=CSW2/2: The relationship between capacity value CSW1 of capacitive elements 410a and 410b and capacity value CSW2 of capacitive elements 420a and 420b. 
As shown in FIG. 10 and Formula 9, the frequency control sensitivity, which is a gradient of VT vs. fOSC of bands 1 to 4, in this conventional local oscillator 10 becomes higher for a band with higher oscillation frequency fOSC. In a band with high frequency control sensitivity, fOSC fluctuates by voltage noise superimposed on control voltage VT, and thus a phase noise of local oscillator 10 degrades.
In recently-popular broadcast receiving systems and communications systems that employ digital modulation, a local oscillator with next features are demanded: A broad oscillation frequency range for handling multiple divided frequency bands, and good phase noise characteristic that does not degrade a bit-error rate even if signals are phase-modulated to multiple values for achieving high-quality image, sound, and data.
Patent Document Japanese Patent No. 3488180